1. Field of the Invention
The invention relates to a method for slicing a multiplicity of wafers from a workpiece by means of a wire saw.
2. Background Art
Wire saws are suitable, for example, for slicing a multiplicity of semiconductor wafers, solar wafers and other crystal wafers from a crystal piece in one working step. The functional principle of such a wire saw is described in U.S. Pat. No. 5,771,876. Wire saws have a wire gang, which is formed by a sawing wire that is wound around two or more wire feed or guide rolls. The sawing wire may be covered with a cutting layer. When using wire saws having a sawing wire without firmly bound cutting abrasive, cutting abrasive in the form of a suspension (slurry) is supplied during the slicing process.
During the slicing process, the workpiece fastened on a table passes through the wire gang, in which the sawing wire is arranged in the form of wire sections lying parallel to one another. The passage through the wire gang is brought about by means of a relative movement between the table and the wire gang, induced by means of a forward feed device, which feeds the workpiece against the wire gang (table forward feed) or the wire gang against the workpiece.
Conventionally, the workpiece is connected on its circumferential surfaces to a sawing beam, into which the sawing wire cuts after having sliced through the workpiece.
The sawing beam may for example be a graphite beam or epoxy beam, which is adhesively bonded or cemented onto the circumferential surface of the workpiece. Lastly, the workpiece with the sawing beam is cemented onto the table.
After the slicing, the sliced wafers remain fixed on the sawing beam like the teeth of a comb and thus can be taken out of the wire saw. Subsequently, the remaining sawing beam is removed from the wafers.
In the prior art, the problem arises during wire sawing that the sawing wire experiences a different degree of wear as a function of the incision length. With respect to the term incision length, reference will be made to FIG. 3 and the associated description.
In order to avoid this, it is proposed in U.S. Pat. No. 6,109,253 A to keep the table forward feed constant during the sawing process and to adapt the (effective) speed of the sawing wire according to the incision length. There is intended to be a proportional relation between the wire speed and the incision length. When slicing semiconductor wafers with sizeable diameters from a crystal, for example 300 mm wafers, a table forward feed being kept constant is however disadvantageous since the total thickness variance (TTV) of the semiconductor wafers is thereby increased in the region of the greatest incision lengths. A constant table forward feed, as disclosed in U.S. Pat. No. 6,109,253 A, is therefore undesirable.
JP 9262826 A on the other hand teaches to vary the table forward feed as a function of the incision length. At the start of the cutting process, this leads to initially increased wire wear which is subsequently reduced by lowering the table forward feed. Ideally the wire wear decreases linearly. The described method thus leads as before to inhomogeneous, albeit linearly decreasing wire wear.